1. Field of the Invention
The present invention relates to a 2 to n optical divider with integrated optics. More particularly, it relates to an optical divider that may find use in the field of optical telecommunications, such as, in the 1260-1360 nm and 1480-1660 nm spectral windows.
2. Description of Related Art
A 2 to n optical divider (where n is an integer greater than or equal to 2) includes at least one optical divider element comprising 2 inputs and 2 outputs to divide a light wave injected into one of the inputs into 2 parts. The light is distributed to each of the outputs with a predefined division ratio.
When n is more than 2, the optical divider comprises several cascaded divider elements so as to have 2 inputs and n outputs and enable the distribution of a light wave injected into one of the two inputs, to the n outputs according to a predefined division ratio for each of the outputs.
U.S. Pat. No. 5,835,651 describes a conventional 2 to 2 divider.
FIG. 1 diagrammatically shows a conventional 2 to 2 divider of this type made with integrated optics, in the xy plane of the substrate containing the divider.
In this figure, the substrate in which the divider is made is not shown. FIG. 1 shows first and second single-mode input wave guides 1 and 3, first and second single-mode output wave guides 5 and 7 and a dual-mode wave guide 9 with length La along the x axis and width Wa along the y axis. The dual-mode wave guide 9 connects the input wave guides and the output wave guides. The input and output wave guides are connected to the dual-mode guide at an angle xcex2 from the x axis.
With this divider, a light wave Ea injected into one of the single-mode input guides, for example guide 1, propagates in the guide in the direction of the dual-mode guide 9 and becomes closer to the second input guide 3, thus, setting up a proximity coupling with the second input guide. This proximity coupling is greater for higher wavelengths (such as wavelengths within the 1480-1660 nm spectral band) than for lower wavelengths (such as wavelengths in the 1260-1360 nm spectral band).
At the end of the single-mode input guides 1, 3, the light wave is coupled to the two modes of the dual-mode guide 9. The spectral behavior of coupling between these two modes during propagation in the dual-mode guide is contrary to the behavior in single-mode guides. In other words, coupling for higher wavelengths (1480-1660 nm) is weaker than for lower wavelengths (1260-1360 nm).
At the output from the dual-mode guide 9, the light wave is coupled with a given distribution onto the two single-mode output guides 5, 7. The light wave is once again affected by proximity coupling, until the single-mode guides have separated by a distance H such that the light wave propagating in each single-mode guide no longer sees the other-guide.
As a result, a light wave Ea is distributed into two light waves S1a, S2a in the two single-mode output guides 5, 7.
The spectral behavior in the single-mode input and output guides that is contrary to the spectral behavior in the dual-mode guide, allows the creation of a 2 to 2 achromatic divider in the dual-mode guide, for selected values of xcex2, Wa and La. For example, a low value of xcex2 limits excess losses.
Although it is satisfactory in some respects, that the light wave in this 2 to 2 divider be affected by a discontinuity at each end of the dual-mode guide connected to the single-mode guides creating mismatch losses between the modes of the dual-mode guide and the modes of the single-mode guides, and reflection losses. These mismatch and reflection losses are particularly annoying for applications in the optical telecommunications field.
Moreover, as shown previously, excess losses and achromatism depend on xcex2. On one hand, xcex2 must increase to reduce chromatism. On the other hand, xcex2 must decrease to reduce excess losses. This behavior of the divider makes it difficult to make a 2 to 2 divider with good achromatism and low excess losses.
An aspect of embodiments of this invention is to provide a 2 to n optical divider with integrated optics without the limitations and difficulties of conventional dividers.
In particular, one aspect of embodiments of the invention is to provide a 2 to n divider with low excess losses and satisfactory achromatism, particularly for optical telecommunications in all of the 1260-1360 nm and 1480-1660 nm spectral windows. The divider, according to one embodiment of the invention, is advantageously very slightly chromatic or even achromatic, and has minimum excess losses.
In the remainder of the description, achromatic refers to either low chromatism (for example  less than 0.5 dB for telecommunications spectral windows) or xe2x80x9cperfectxe2x80x9d achromatism.
A further aspect of the invention is to make a 2 to n divider in which excess losses and chromatism are independent to facilitate its application.
Another aspect of the invention is to provide a 2 to n divider without any discontinuities for the light wave so as to limit mismatch and reflection losses.
In one embodiment of the invention, the 2 to n divider with integrated optics, with n being an integer greater than or equal to 2, comprises at least one 2 to 2 optical divider, element in a substrate. This element comprises a first and a second guide, with widths equal to W1 and W2 respectively, suitable for dividing an input light wave E input in one of the guides, into a first and second output wave S1 and S2 transported by the first and second guides, respectively. These first and second guides have at least three parts:
a first part, of a first coupling type, in which the first and second guides progressively move towards each other, until a distance Dc that is not zero and is less than a threshold distance Ds corresponding to the minimum distance starting from which the input light wave input into one of the guides can be at least partly coupled in the other guide,
a second part, of a second coupling type, with length Lc, called the coupling length, in which the guides are approximately parallel to each other and are distant by the value Dc, and
a third part, of a first coupling type, in which the guides gradually separate starting from the value Dc until they are separated by a value of more, than Ds.
The values Dc, Lc, W1 and W2 are chosen so as to obtain an achromatic divider element at the divider operating wavelengths. The values Dc and Lc are chosen so that the first coupling type and the second coupling type vary inversely with the wavelengths.
The light wave is divided with a division ratio CR related to the output of one of the first or second guides (by convention).
In the present description, an optical guide is a guide with lateral confinement, unlike a planar guide in which light can propagate within a plane (the guide plane).
The optical guides according to one embodiment of the invention are preferably single-mode.
An optical guide is composed of a central part generally called the core and media surrounding the core that may be identical to each other or different from each other.
To enable confinement of light in the core, the refraction index of the medium from which the core is made must be different and in most cases greater than the refraction index of the surrounding media.
To simplify that description, the guide will be considered to consist of the central part of the core. Furthermore, all or part of the surrounding media will be called the substrate. However, one of ordinary skill in the art would understand that when the guide is not buried or is only slightly buried, one of the surrounding media may be outside the substrate and may for example be air.
The substrate may be a single-layer or a multi-layer, depending on the type of technique used.
Furthermore, depending on the application, an optical guide in a substrate may be more or less buried in the substrate and particularly may comprise portions of the guide buried at variable depths. This is particularly true in the technology for ion exchanges in glass.
According to one embodiment, the divider is made with integrated optics in a glass substrate using ion exchange techniques.
According to one embodiment of the invention, dedicated particularly to telecommunications applications, the first and the second guides have widths W1 and W2 such that the behavior of the divider element based on Dc and Lc is achromatic in the operations spectral windows from 1260 to 1360 nm and from 1480 to 1660 nm.
We will usually choose W1=W2=W.
Preferably, the guides become closer to each other and/or separate from each other symmetrically.
Excess losses C may be defined using the following equation:
C=10 log (PS1+PS2)/PE
where PS1, PS2, PE are the powers of waves S1, S2 and E, respectively.
The choice of values Dc, Lc, W1 and W2 provides a means for compensating for light coupling phenomena between the two guides that are different depending on the wavelengths, and thus create an achromatic divider element.
When the distance D between the guides is more than the value Ds, there is no coupling between the guides. When the distance D between the guides is between the distance Ds and a distance Dx, the guides set up a weak proximity coupling that is greater for high wavelengths (for example 1480-1660 nm) than for low wavelengths (for example 1260-1360 nm). On the other hand, when the distance D between the guides becomes smaller and is between Dx and Dc, the operating conditions are changed and the phenomenon involved is then a strong proximity coupling that is weaker for higher wavelengths (for example 1480-1660 nm) than for lower wavelengths (for example 1260-1360 nm). This strong proximity coupling is made particularly on the length Lc of the second part.
Furthermore, since Dc is not zero, the light wave is not affected by any discontinuity in this divider element, which results in very small excess losses. In one preferred embodiment, Dc must be greater than Dmin, where Dmin=0.5 xcexcm.
The value Dx can be defined as being the distance separating the two guides starting from which the proximity coupling is inverted from strong to weak and vice versa.
The 2 to 2 divider element made according to the invention may be considered as comprising two types of proximity couplers; a first type of coupler operating globally in the weak coupling conditions, corresponding to parts I and III of the divider itself if strong coupling zones can exist in these parts, depending on the value of Dx; and a second type of coupler operating in the strong coupling condition corresponding to part II of the divider.
Moreover, in parts I and III of the divider element corresponding to weak coupling, the guides can be brought close to each other and/or separated from each other moving along an arc of a circle with a radius R greater than Rc. The value Rc is defined as being the critical radius of curvature above which there are no longer any curvature losses at the highest wavelength in the spectral window considered (for example 1260-1360 nm and 1480-1660 nm). This value Rc is defined in order to minimize excess losses of the 2 to 2 divider element.
In one of the embodiments, the radius R will be taken equal to Rc in order to minimize weak proximity coupling.
Furthermore, as R becomes smaller, the divider element will become more compact. Therefore there is a two-fold advantage in choosing R=Rc.
In the case of a 2 to n divider with integrated optics, where n is an integer greater than 2, this divider comprises a 2 to 2 optical divider element in the substrate like the one described above, and nxe2x88x922 cascaded 1 to 2 divider elements such that the divider comprises 2 inputs corresponding to the 2 to 2 divider element input guides and n outputs.
The 1 to 2 divider elements are selected from among Y couplers or junctions These divider elements may or may not be symmetric.
An asymmetric divider element may be obtained in case of a coupler, by varying the coupler interaction length and/or the selection of the different coupler output channels.
An asymmetric divider element may be obtained in the case in which a Y junction is used by varying the section of the output channels from the junction and/or the angle between the output channels from the junction and the optical axis of the junction input channel.
Other specific features and aspects of the invention will become apparent when taken with the detailed description and examining the attached drawings.